Gun-shot residue plays an important role in forensic science in helping to determine certain factors of a shooting and/or related criminal cases. Gun-shot residue (GSR) is caused by the combustion involved in the firing of ammunition. When a gun is fired, the trigger of the gun is pulled causing a firing pin to strike the ammunition (i.e., bullet), crushing the primer. The energy transfer causes the explosion of the gun powder sending the bullet through the barrel. The velocity of the bullet is stabilized by a spiraling motion caused by lands and grooves in the barrel called riflings. In a crime laboratory, the riflings are used to match a bullet to a particular gun provided the bullet-shell is found at the crime scene. GSR is not only created as a cloud in the direct vicinity of the gun, but GSR is also propelled in the wake of the bullet in the direction of the target. GSR is obtained from every shooting incident and needs to be analyzed.
One conventional test for analyzing GSR is a chemical test, called the Modified Griess Test. The Modified Griess Test is a test to detect the presence of nitrite residues, and is the primary test used by firearms examiners to determine a muzzle-to-garment distance. The Modified Griess Test is performed first on the GSR since the test will not interfere with later tests for lead residues. Nitrite residues are a byproduct of the combustion of smokeless gunpowder. When a gun is discharged, nitrite particles are expelled from the muzzle of a gun and can be imbedded in, or deposited on, the surface of a target. Another conventional test conducted on GSR is called the Sodium Rhodizionate Test, which is a chemical test designed to determine if lead residues are present on the exhibit.
A problem with both the Modified Griess Test and the Sodium Rhodizionate Test is that most shooting cases involve firing at close range, and these tests are not applicable to shootings at close ranges (e.g., less than 5 feet). These techniques can only observe microscopic particles (particles whose diameter is a few microns or more) that are formed at distances of 5 feet or longer from the gun. Currently the GSR patterns are experimentally matched with the patterns at the crime scene on test firing. This is a time consuming and expensive process, and, again, does not work for short distances since it is difficult to observe a pattern in such a short distance. Moreover, these techniques require substantial amounts of GSR samples, which are difficult to obtain and are frequently contaminated. Accordingly, conventional techniques used for GSR analysis are limited, so prosecuting and defense attorneys typically rely on other evidence such as cartridge case volume and witness testimony to build a given case.
Accordingly, what is needed is an improved method and system for analyzing gun-shot residue. The present invention addresses such a need.